.. PRTG Manual: REST Custom Sensor

The REST Custom sensor queries a REST Application Programming Interface (API) endpoint and maps the JSON or XML result to sensor values. The mapping rule has to be available as a REST configuration file in JSON template (*.template) format according to the PRTG API definition for custom sensors.

The sensor can show values returned by a REST API in multiple channels.

For details about the return value format, see section Custom Sensors.

The REST configuration file that contains the mapping rules must be stored on the system of the probe the sensor is created on: If used on a remote probe, the file must be stored on the system running the remote probe. In a cluster setup, please copy the file to every cluster node.

This sensor type does not support more than 50 channels officially. If you have more than 50 channel mappings in your REST configuration file, the sensor will show an error.

In a cluster, status changes triggered by limits only work on the master node.

Currently, this sensor type is in beta status. The methods of operating can change at any time, as well as the available settings. Do not expect that all functions will work properly, or that this sensor works as expected at all. Be aware that this type of sensor can be removed again from PRTG at any time.

Limited to 50 Sensor Channels

PRTG does not officially support more than 50 sensor channels. Depending on the data used with this sensor type, you might exceed the maximum number of supported sensor channels. In this case, PRTG will try to display all sensor channels. However, please be aware that you will experience limited usability and performance.

Add Sensor

The Add Sensor dialog appears when you manually add a new sensor to a device. It only shows the setting fields that are required for creating the sensor. Therefore, you will not see all setting fields in this dialog. You can change (nearly) all settings in the sensor's Settings tab later.

The following settings for this sensor differ in the Add Sensor dialog in comparison to the sensor's Settings tab.

REST Specific

REST Configuration

Select a REST configuration from the list. The sensor will use it to map the JSON or XML result from the REST query into sensor values.

Default is channelDiscovery. If you select this REST configuration, the sensor will parse the returned JSON or XML and automatically create channels based on available values: one channel for each number and boolean, for strings if they are convertible into numbers.

This list shows all available files in the \Custom Sensors\rest sub-directory of the probe system's PRTG program directory (see Data Storage). To appear in this list, store the files as JSON template (*.template) in this folder.

Using custom scripts is not possible on hosted probes. To use custom REST configurations, add this sensor to a remote probe instead and save the file on this system.

To show the expected values and sensor status, your files must return the expected JSON format. Channels, values, and messages must be embedded in valid JSON using JSONPath. For details, see section Define Sensor Channels and Select Channel Values.

For detailed information on how to build custom sensors and for the expected return format, see section Custom Sensors for more information.

There are several REST configuration files available by default. They are ready to use. You can also analyze them to see how to write your own REST configuration.

When using custom sensors on the Cluster Probe, please copy your files to every cluster node installation.

Sensor Settings

On the details page of a sensor, click the Settings tab to change its settings.

Usually, a sensor connects to the IP Address or DNS Name of the parent device where you created this sensor. See the Device Settings for details. For some sensor types, you can define the monitoring target explicitly in the sensor settings. See below for details on available settings.

Enter one or more Tags, separated by spaces or commas. You can use tags to group sensors and use tag–filtered views later on. Tags are not case sensitive. We recommend that you use the default value.

There are default tags that are automatically predefined in a sensor's settings when you add a sensor. See section Default Tags below.

You can add additional tags to the sensor if you like. Other tags are automatically inherited from objects further up in the device tree. These are visible above as Parent Tags.

It is not possible to enter tags with a leading plus (+) or minus (-) sign, nor tags with round parentheses (()) or angle brackets (<>).

Priority

Select a priority for the sensor. This setting determines where the sensor is placed in sensor lists. Top priority is at the top of a list. Choose from one star (low priority) to five stars (top priority).

Default Tags

restcustomsensor, restsensor

REST Specific

Timeout (Sec.)

Enter a timeout in seconds for the request. If the reply takes longer than this value defines, the sensor will cancel the request and show a corresponding error message. Please enter an integer value. The maximum value is 900 seconds (15 minutes).

Request Method

Choose an HTTP request method to determine how the sensor will request the REST API.

GET (default): Use the GET method to request the REST API.

POST: Use the POST method to request the REST API.

If you use the POST method, please use the content type, for example application/x-www-form-urlencoded, as HTTP header. In section HTTP Headers, choose Use custom HTTP headers and enter the content type into the Custom HTTP Headers field.

Postdata

This field is only visible when you select the POST request method above. Enter the data part for the POST request here.

If you use the POST method, please use the content type, for example application/x-www-form-urlencoded, as HTTP header. In section HTTP Headers, choose Use custom HTTP headers and enter the content type into the Custom HTTP Headers field.

Request Protocol

Define the security of the HTTP request. Choose between:

HTTP (default): Send the REST query via insecure HTTP.

HTTPS: Send the REST query via secure HTTPS.

Authentication Method

Select the authentication method for access to the REST API. Choose from:

No authentication (default): Do not use any authentication for the request.

Basic authentication with Windows credentials from parent device: Use the Windows credentials from the parent device. See section Inheritance of Settings for more information.

Token: Use a JSON Web Token (JWT) or OAuth2 Bearer Token for authentication.

User

This field is only visible if you enable basic authentication above. Enter a username for the REST API. Please enter a string.

Password

This field is only visible if you enable basic authentication above. Enter a password for the REST API. Please enter a string.

Token

This field is only visible if you enable token authentication above. Enter a JWT or OAuth2 Bearer Token that is required by the REST API for authentication.

HTTP Headers

Define if you want to send custom HTTP headers to the target URL. Choose between:

Do not use custom HTTP headers

Use custom HTTP headers

Custom HTTP Headers

This field is only available if you select using custom headers above. Enter a list of custom HTTP headers with their respective values that you want to transmit to the URL you define above, each pair in one line. The syntax of a header-value pair is header1:value1.

If you enter more than one header-value pair, they must be separated by delimiters. The syntax is header1:value1|header2:value2|header3:value3

The sensor does not support the header field names user-agent, content-length, host.

Ensure that the HTTP header statement is valid! Otherwise, the sensor request will not be successful.

If you selected the POST Request Method setting above, enter the content type, for example application/x-www-form-urlencoded, as custom HTTP header.

REST Query

This field shows the REST query that this sensor executes. To change it, enter a valid query for the target REST API. The syntax is: [:port]/path[?var1=val1&...]

The first part of the address is always inherited from the parent device's address. Only enter the path to the REST endpoint of the parent device. You can override the port if necessary.

For example, if you add the sensor to a probe device, a query to the REST API of your PRTG installation that returns the number of sensors on the probe would look like this:

Shows the REST configuration file that the sensor uses to map returned JSON or XML into sensor values. Once a sensor is created, you cannot change this value. It is shown for reference purposes only. If you need to change this, please add the sensor anew.

Debug Options

Sensor Result

Define what PRTG will do with the sensor results. Choose between:

Discard sensor result: Do not store the sensor result.

Write sensor result to disk (Filename: Result of Sensor [ID].txt): Store the last result received from the sensor to the Logs (Sensors) directory in the PRTG data folder on the probe system the sensor is running on (on the Master node if in a cluster). File names: Result of Sensor [ID].txt and Result of Sensor [ID].Data.txt. This is for debugging purposes. PRTG overwrites these files with each scanning interval. For more information on how to find the folder used for storage, see section Data Storage.

This option is not available when the sensor runs on the Hosted Probe of a PRTG hosted by Paessler instance.

Sensor Display

Primary Channel

Select a channel from the list to define it as the primary channel. In the device tree, the last value of the primary channel will always be displayed below the sensor's name. The available options depend on what channels are available for this sensor.

You can set another primary channel later by clicking the pin symbol of a channel in the sensor's Overview tab.

Graph Type

Define how different channels will be shown for this sensor.

Show channels independently (default): Show an own graph for each channel.

Stack channels on top of each other: Stack channels on top of each other to create a multi-channel graph. This will generate an easy-to-read graph that visualizes the different components of your total traffic. This option cannot be used in combination with manual Vertical Axis Scaling (available in the Sensor Channel Settings settings).

Stack Unit

This setting is only available if stacked graphs are selected above. Choose a unit from the list. All channels with this unit will be stacked on top of each other. By default, you cannot exclude single channels from stacking if they use the selected unit. However, there is an advanced procedure to do so.

Inherited Settings

By default, all following settings are inherited from objects higher in the hierarchy and should be changed there, if necessary. Often, best practice is to change them centrally in the Root group's settings, see section Inheritance of Settings for more information. To change a setting only for this object, disable inheritance by clicking the button next to inherit from under the corresponding setting name. You will then see the options described below.

Select a scanning interval (seconds, minutes, or hours) from the list. The scanning interval determines the time the sensor waits between two scans. You can change the available intervals in the system administration on PRTG on premises installations.

If a Sensor Query Fails

Define the number of scanning intervals that a sensor has time to reach and check a device again in case a sensor query fails. The sensor can try to re-reach and check a device several times, depending on the option you select here, before it will be set to a Downstatus. This helps you avoid false alarms if the monitored device has only temporary issues. For previous scanning intervals with failed requests, the sensor will show a Warning status. Choose from:

Set sensor to down immediately: The sensor will show an error immediately after the first failed request.

Set sensor to warning for 1 interval, then set to down (recommended): After the first failed request, the sensor will show a warning status. If the following request also fails, the sensor will show an error.

Set sensor to warning for 2 intervals, then set to down: Show an error status only after three consecutively failed requests.

Set sensor to warning for 3 intervals, then set to down: Show an error status only after four consecutively failed requests.

Set sensor to warning for 4 intervals, then set to down: Show an error status only after five consecutively failed requests.

Set sensor to warning for 5 intervals, then set to down: Show an error status only after six consecutively failed requests.

Sensors that monitor via Windows Management Instrumentation (WMI) always wait at least one scanning interval until they show an error. It is not possible to set a WMI sensor to Down immediately, so the first option will not apply to these sensor types. All other options can apply.

If a sensor has defined error limits for channels, it will always show a Down status immediately, so no "wait" option will apply.

If a channel uses lookup values, it will always show a Down status immediately, so no "wait" options will apply.

Schedules, Dependencies, and Maintenance Window

Inheritance for schedules, dependencies, and maintenance windows cannot be interrupted. The corresponding settings from the parent objects will always be active. However, you can define additional settings here. They will be active at the same time as the parent objects' settings.

Schedule

Select a schedule from the list. Schedules can be used to monitor for a certain time span (days, hours) every week. With the period list option it is also possible to pause monitoring for a specific time span. You can create new schedules and edit existing ones in the account settings.

Schedules are generally inherited. New schedules will be added to existing ones, so all schedules are active at the same time.

Maintenance Window

Specify if you want to set up a one-time maintenance window. During a "maintenance window" period, this object and all child objects will not be monitored. They will be in a Paused status instead. Choose between:

Not set (monitor continuously): No maintenance window will be set and monitoring will always be active.

Set up a one-time maintenance window: Pause monitoring within a maintenance window. You can define a time span for a monitoring pause below and change it even for a currently running maintenance window.

To terminate a current maintenance window before the defined end date, change the time entry in Maintenance Ends field to a date in the past.

Maintenance Begins

This field is only visible if you enabled the maintenance window above. Use the date time picker to enter the start date and time of the maintenance window.

Maintenance Ends

This field is only visible if you enabled the maintenance window above. Use the date time picker to enter the end date and time of the maintenance window.

Dependency Type

Define a dependency type. Dependencies can be used to pause monitoring for an object depending on the status of another. You can choose from:

Use parent: Pause the current sensor if the device, where it is created on, is in Down status, or is paused by another dependency.

Select object: Pause the current sensor if the device, where it is created on, is in Down status, or is paused by another dependency. Additionally, pause the current sensor if a specific other object in the device tree is in Down status, or is paused by another dependency. Select below.

Master object for parent: Make this sensor the master object for its parent device. The sensor will influence the behavior of the device, where it is created on: If the sensor is in Down status, the device will be paused. For example, it is a good idea to make a Ping sensor the master object for its parent device to pause monitoring for all other sensors on the device in case the device cannot even be pinged. Additionally, the sensor will be paused if the parent group of its parent device is in Down status, or if it is paused by another dependency.

Testing your dependencies is easy! Simply choose Simulate Error Status from the context menu of an object that other objects depend on. A few seconds later all dependent objects should be paused. You can check all dependencies in your PRTG installation by selecting Devices | Dependencies from the main menu bar.

Dependency

This field is only visible if the Select object option is enabled above. Click on the reading-glasses and use the object selector to choose an object on which the current sensor will depend.

Dependency Delay (Sec.)

Define a time span in seconds for a dependency delay. After the master object for this dependency goes back to Up status, PRTG will start monitoring the depending objects after this extra delayed. This can help to avoid false alarms, for example, after a server restart, by giving systems more time for all services to start up. Please enter an integer value.

This setting is not available if you choose this sensor to Use parent or to be the Master object for parent. In this case, please define delays in the parent Device Settings or in the superior Group Settings.

Define which user group(s) will have access to the object you're editing. A table with user groups and types of access rights is shown. It contains all user groups from your setup. For each user group, you can choose from the following access rights:

Inherited: Use the access rights settings of the parent object.

None: Users in this group cannot see or edit the object. The object neither shows up in lists nor in the device tree. Exception: If a child object is visible to the user, the object is visible in the device tree, though not accessible.

Read: Users in this group can see the object and review its monitoring results.

Write: Users in this group can see the object, review its monitoring results, and edit the object's settings. They cannot edit access rights settings.

Full: Users in this group can see the object, review its monitoring results, edit the object's settings, and edit access rights settings.

You can create new user groups in the System Administration—User Groupssettings. To automatically set all objects further down in the hierarchy to inherit this object's access rights, set a check mark for the Revert children's access rights to inherited option.

For each type of sensor channel, define the unit in which data is displayed. If defined on probe, group, or device level, these settings can be inherited to all sensors underneath. You can set units for the following channel types (if available):

Bandwidth

Memory

Disk

File

Custom

Custom channel types can be set on sensor level only.

Define Sensor Channels and Select Channel Values

In your REST configuration file, you have to define which values of the returned JSON or XML will be mapped to which sensor channels.

A sensor channel is defined by the channel key in your REST configuration. See the JSON Return Format: Minimum Example in section Custom Sensors.

A channel value is defined by an expression that retrieves and processes the value from the JSON source. The expression can contain JSONPath, gval operators, and functions.

Example

For this example, we take PRTG as REST endpoint and query the sensor status statistics for the local probe. The REST query that we enter in the sensor settings looks like this:

Your REST configuration has to translate this JSON for the sensor. It has to be available as JSON template (*template) in the \Custom Sensors\rest sub-directory of your PRTG probe system. See section Custom Sensors for details about the JSON return format.

The following example returns two channels from the JSON resource that will be added to the sensor in PRTG, Total (total sensor count) and Alarms (sensors in error status), each selected by their keys in the returned JSON.

The channel values will be the values of the corresponding properties of the REST result defined in JSONPath notation, $.sensorxref[0].totalsens (28) and $.sensorxref[0].downsens (0).

Each value's property (destination) and the text property is set to the appropriate transformation rules from source to destination. This is the JSON path of the source. The sensor replaces each path with the value from the source.

There are several REST configuration files available in the \Custom Sensors\rest folder by default. They are ready to use. You can also analyze them to see how to write your own REST configuration.

JSONPath

The REST Custom sensor uses JSONPath to assign values from the returned JSON to sensor channels. With JSONPath you provide the path to the value in the JSON source that you want to monitor in a sensor channel.

The JSONPath implementation that PRTG uses for the REST Custom sensor might differ from other JSONPath implementations. You can test and debug your definitions using the command line. To test simple JSONPath expressions and calculations, you can also use JSONPath Online Evaluator, for example. Please note that this tool might not work properly with complex JSONPath expressions that PRTG supports.

Example

To demonstrate the practical usage of JSONPath, we use this JSON example that a REST query might have returned as reference in this section.

.<key> must only contain letters, numbers, and underscore (_). Hyphens (-) are not supported.

[<key>] must only contain a number or a quoted string.

Example

This expression matches 35985021 in the example above:

$.devices.0.networks.a.rx_bytes

You get the same result with this expression:

$["devices"][0]["networks"]["a"]["rx_bytes"]

If an element contains a hyphen (-), the .<key> notation does not work. Use the [<key>] notation in this case:

$["data"][0]["system-stats"]["temps"]["Board (CPU)"]

Wildcard

To match multiple values, you can use the asterisk symbol (*).

.*

[*]

Example

This expression matches 35985021 and 40085321 in the example above:

$.devices[0].networks.*.rx_bytes

Recursive Descent

You can match all sub-items of a node with two dots (..).

Example

This expression matches 7229493 and 55294975 and 7229472 in the example above:

$..tx_bytes

Union

You can match multiple children with [<key1>,<key2>,<...>].

Example

This expression matches 35985021 and 7229493 in the example above:

$.devices.0.networks.a["rx_bytes","tx_bytes"]

Slice

You can match multiple children of an array with [<begin>:<end>] or [<begin>:<end>:<step>].

By default, begin, end, and step are either integers or empty.

The default approach is to go step by step from the first array element to the last element.

Step can also be a negative integer to go through the array in reversed order.

Example

This expression matches 63685865 in the example above:

$.devices[-1:].networks.a.rx_bytes

Current

The @ symbol matches the current element.

Example

This expression matches to 40.085321 and 55.294975 in the example above and can be used to receive a percentage value:

$.devices[1].networks.a.["rx_bytes","tx_bytes"](@/100000000*100)

Filter

You can filter matches with [?<expression>].

Example

This expression matches 35985021 in the example above because the first device is the only one with a beta channel:

$.devices[?@.firmware.channel=="beta"].networks.a.rx_bytes

Script

You can modify matches with (<expression>) and keys with [<expression>].

Example

This expression matches true and false in the example above because only the first device has a beta channel:

$.devices[*](@.firmware.channel=="beta")

Placeholder

Placeholders give access to values of wildcards in a match. A placeholder #n (where n is a natural number) represents the value of the nth wildcard. You can use this in the keys of JSON arrays.

Example

This expression creates a JSON map from ids to the corresponding firmware channel and matches {"0.7":"beta"} in the example above:

{$.devices[#0].id:$.devices[*].channel}

This is an extension of the official JSONPath.

Constant

Numeric constants as 64-bit floating point: 12345.678

String constants with double quotes: "switch"

Boolean constants: true and false

Operator

Parentheses control the order of evaluation: (<expression>)

Arrays with square brackets: [<expression>, <expression>, ...]

Objects: {<expression>:<expression>, <expression>:<expression>, ...}

Conditionals: <condition> ? <expression> : <expression>

Calculating Channel Values and Functions

You can perform calculations with the values of the source JSON or XML. The sensor uses the Paessler gval package for the calculations. Please refer to the gval documentation for details.

The following operators and functions are available to calculate channel values.

REST Custom Sensor: Operators

Operator

Description

Operand Type

Output Type

Infix Modifiers

+

plus

number

number

-

minus

number

number

*

times

number

number

**

power of

number

number

%

modulo

number

number

/

divide

number

number

&

bitwise and

number

number

|

bitwise or

number

number

^

bitwise xor

number

number

<<

shift left

number

number

>>

shift right

number

number

Logical Infix Operators

>

greater than

number/string

bool

>=

equal or greater than

number/string

bool

<

less than

number/string

bool

<=

equal or less than

number/string

bool

==

equal

any

bool

!=

not equal

any

bool

=~

match regular expression

string

bool

!~

mismatch regular expression

string

bool

in

contains element

any, array

bool

&&

and

and

bool

||

or

or

bool

??

coalescence

any

any

Prefix Operators

-

negative

number

number

~

bitwise not

number

number

!

not

bool

bool

REST Custom Sensor: Functions

duration(start,end)

Calculates the nanoseconds between start and end.

Both parameters must be RFC3339 date time strings.

Example

duration($.devices[0].firmware.date, "017-05-18T17:11:43.7049944Z")

now()

Returns the current date time in RFC3339.

Example

duration($.devices[0].firmware.date, now())

number(string, [base])

Converts a string to a floating point number.

If the base is not set, it will be detected via the prefix of the string.

"0": base = 8

"0X": base = 16

otherwise: base = 10

Decimals are only supported at base 10.

Examples

number("10.5")number("a", 16)

len(object/array/string)

Returns the length of an array or string and counts the number of properties in a JSON object.

Example

len($..(number(@)))

This expression counts every number or string that can be converted into a number. It returns 13 in the example above.

sum(array-/object-of-numbers)

Returns the sum over an array of numbers.

Example

sum([1,2,3])

This expression returns 6.

mean(array-/object-of-numbers)

Returns the average value of an array of numbers.

Example

mean([1,2,3])

This expression returns 2.

lookup(string, string, string, ...)

Returns the index of the given string in a string list, or -1 if the string is not found.

Example

lookup($.device[0].firmware.channel, "stable", "beta", "alpha")

This expression returns 1 because $.device[0].firmware.channel resolves to beta.

implode(array-/object-of-string, string)

Returns the concatenation of each string in the array, separated by the given string.

Example

implode($..tx_bytes, ",")

This expression returns 7229493,7229472.

Generic Channels

You can create a template in your REST configuration that defines generic channels. Generic channels are created based on the data that the REST endpoint returns. When the returned value is an array or object, the sensor will create a channel for each element and concatenate the corresponding key to the channel name.

Example

Imagine you want to have a total byte channel for each network card that is defined in the JSON example above. You can do this by creating a dynamic channel like in the following example.

Every channel must have unique name. Channels cannot be removed once they are added and will keep their last received value. New channels can be added.

XML Sources

If the REST source returns XML instead of JSON, the sensor will transform the XML result to JSON before replacing the value paths with source values. Because of this, you will not know the structure of the source JSON to correctly provide the paths.

In this case, manually execute the sensor executable rest.exe from the \Sensor System subfolder of the PRTG probe system. Execute rest.exe with the address of the XML endpoint and the parameter passthrough. The rest executable will return the converted XML to JSON result that you can use to define the desired paths.

rest.exe <xml-endpoint-url> passthrough

Usage and Debugging

To create a suitable REST configuration for the sensor, you might want to check the returned JSON or XML and see what happens when your mapping rules apply.

The REST Custom sensor is an EXE sensor, so you can test and debug your configuration by executing rest.exe with several parameters. The rest.exe is located in the \Sensor System subfolder of the PRTG program directory.

Command syntax:

rest.exe url template|passthrough|channelDiscovery [flags]

REST.exe: Parameters

url

Address of the REST API endpoint that returns JSON or XML

template

Fully qualified path and file name of your REST configuration file used to map the JSON result to the sensor

passthrough

No mapping, only returning the queried JSON or XML

Useful to analyze XML that has been converted to JSON

channelDiscovery

Creates a channel for every number or boolean in the returned JSON or XML

If possible, it converts string values to number or boolean values.

REST.exe: Flags

-authtoken <string>

JWT or OAuth2 Bearer Token to send with the request in authorization header as Bearer